1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking electrolyte electrode assemblies and separators alternately. Each of the electrolyte electrode assemblies includes a pair of electrodes and an electrolyte interposed between the electrodes.
2. Description of the Related Art
In recent years, various types of fuel cells such as a solid polymer electrolyte fuel cell have been developed. The solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the electrodes comprises a catalyst and a porous carbon paper. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of the fuel cell (unit cell) for generating electricity. A predetermined number of unit cells are connected together to form a fuel cell stack.
In the fuel cell stack, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Typically, the fuel cell stack has reactant gas supply passages for supplying the oxygen-containing gas and the fuel gas (reactant gases) to the anode and the cathode, and reactant gas discharge passages for discharging the oxygen-containing gas and the fuel gas from the anode and the cathode. The reactant gas supply passages include an oxygen-containing gas supply passage and a fuel gas supply passage extending in a stacking direction of the fuel cell stack. The reactant gas discharge passages include an oxygen-containing gas discharge passage and a fuel gas discharge passage extending in the stacking direction.
The water produced in the electrochemical reaction on the power generation surfaces of the electrodes flows into the oxygen-containing gas discharge passage, and the water may be trapped or retained in the oxygen-containing gas discharge passage. Further, water vapor condensed into liquid water may be trapped in the fuel gas discharge passage. Therefore, the oxygen-containing gas discharge passage and the fuel gas discharge passage are likely to be narrowed or closed by the trapped water undesirably. Thus, power generation performance of the fuel cells may be degraded due to the insufficient flows of the oxygen-containing gas and the fuel gas.
In an attempt to address the problem, a fuel cell stack is disclosed in the Japanese Laid-Open patent publication No. 2001-236975. In the fuel cell stack, an oxygen-containing gas supply passage connected to a supply port and an oxygen-containing gas discharge passage connected to a discharge port extends in the stacking direction of the fuel cell stack. A bypass plate is provided at a position remote from the discharge port of the oxygen-containing gas discharge passage. A bypass passage formed on the bypass plate has an inlet port connected to the oxygen-containing gas supply passage at one end, and has an outlet port connected to the oxygen-containing gas discharge passage at the other end.
In the fuel cell stack, some of the oxygen-containing gas supplied to the oxygen-containing gas supply passage flows into the inlet port of the bypass passage, flows out of the outlet port of the bypass passage, and jetted into the oxygen-containing gas discharge passage. The water which is produced in the electrochemical reactions is likely to be trapped in the oxygen-containing gas discharge passage at locations remote from the discharge port. The oxygen-containing gas jetted from the bypass passage pushes the trapped water out of the discharge port of the oxygen-containing gas discharge passage. Thus, the water (condensed water) is discharged from the fuel cell stack smoothly, and the desirable power generation performance of the fuel cell stack is maintained.
When the operation of the fuel cell stack is started, or when the operation of the fuel cell stack is restarted after a temporal interruption, condensed water may be present in the pipes for supplying the reactant gases (oxygen-containing gas and/or fuel gas) to the fuel cell stack.
In particular, the amount of condensed water present in the pipe for supplying the oxygen-containing gas to the fuel cell stack tends to be large, and the condensed water may flow into the fuel cell stack undesirably. If droplets of the condensed water cover power generation surfaces of unit cells near the supply ports of the reactant gases, the power generation performance of the unit cells may be degraded undesirably.
In the fuel cell stack, the unit cells are electrically connected in series. Therefore, the respective unit cells generate the same amount of electricity. If voltage drop occurs in some unit cells, the system is operated to limit the amount of electricity collected from the fuel cell stack for protecting the unit cells. If the voltage drop continues in the unit cells, a reverse current may be generated due to a large current beyond the outputting capability of the unit cells.